The present invention relates to a method for coating thermionic emission material for a thermionic emission filament, and particularly to a method for coating thermionic emission material for a thermionic emission filament used in a fluorescent tube.
The conventional filament used in a fluorescent tube is coated with a carbonate oxide of the electron emission material on tungsten wire and is made to emit thermions from the carbonate oxide coating on the tungsten wire due to the generation of heat in the tungsten wire.
In FIG. 1, the coating device is illustrated which exemplifies conventional thermionic emission material coating method for coating carbonate oxide onto a tungsten wire filament.
A conventional carbonate oxide coating device has a carbonate oxide storage tank 6, a supply roller 3 which supplies the tungsten wire used for the filament material and a take up roller 4 which reels up the tungsten wire from supply roller 3 between tank 6 and supply roller 3, there is provided a back tension regulation means 5 which regulates back tension applied to the tungsten wire 31 being reeled onto take-up roller 4. Also, between tank 6 and lake up roller 4 there is provided a drying means to dry the carbonate oxide coated to the tungsten wire.
In such a conventional thermionic emission material coating device, the supply roller 3 is installed to a fixed rotating axle so that it can passively rotate by a tension applied on the tungsten wire. The take-up roller 4 is installed so that rotation is actuated by an accompanying rotation device used to take-up the tungsten wire. Back tension regulation means 5 is a pair of rubber rollers of which number of rotations is regulated in accordance with the tensile strength of the tungsten wire 31. A electric heater is used as drying means 7. Furthermore, the tank 6, which contains dryable carbonate oxide, must be designed to enable immersion of the tungsten wire 31 supplied from supply roller 3 in the carbonate oxide.
Carbonate oxide coating through such a conventional carbonate oxide coating device 1 is processed as follows. The tungsten wire 31 from the supply roller 3 sequentially passes through back tension regulation means 5, dryable carbonate oxide storage tank 6 and drying means 7 when take-up roller 4 rotates.
During such sequential conveyance process, the tungsten wire is coated with carbonate oxide and dried in such a state that a certain tension is applied by the back tension regulation means 5, and finally wound onto the take-up reel 4.
According to this conventional method, since the carbonate oxide coating and drying thereof on the tungsten wire 31 are performed between supply roller 3 which is fixedly positioned and take-up roller 4, the carbonate oxide coating on tungsten wire 31 is not of uniform thickness around the circumference surface of the tungsten wire as shown in FIGS. 2A and 2B.
This non-uniformity of the carbonate oxide coating thickness is caused by running down of the carbonate oxide from the tungsten wire surface due to its weight before drying and after being coated on the tungsten wire during the carbonate oxide coating process. Since the electrical load on the tungsten wire 31 is not applied uniformly from an electric source due to the non-uniform thickness of the carbonate oxide, the carbonate oxide layer coating on the circumference of the tungsten wire peels or flakes off or even breaks.